As is known, a machine for making absorbent sanitary articles has a production line along which advances a continuous web of absorbent material consisting of a layer of permeable material (of non-woven fabric) laid over a layer of impermeable material, with absorbent padding interposed between the two layers. As the continuous web advances along the production line, additional components are applied to the continuous web, such as, for example, lateral stretch bands. lateral sealing flaps, a rear stretch tape and a front band designed to engage the lateral sealing flaps.
Once these additional components have been applied to the continuous web, a continuous succession of absorbent sanitary articles is formed and a cutting device located downstream of the production line divides the continuous succession into individual absorbent articles which are then folded and packed.
Every line for the production of absorbent sanitary articles also comprises at least one rejection station, located downstream of the cutting device, which rejects the defective absorbent sanitary articles, that is to say, the absorbent articles which do not meet specified quality parameters.
Checking for defective absorbent sanitary articles is carried out by a software logic built into an electronic controller of the making machine.
The machine controller, besides generating a machine sync signal 100, is designed to assign information about the article being processed to a shift register 101, as illustrated in FIG. 1. The shift register 101 is defined by a plurality of steps 102, each associated with a time position of an absorbent article being processed along the production line.
In order to identify the defective absorbent articles, the production line comprises a plurality of check points for checking that the absorbent sanitary article has been made up correctly. These check points are connected to the machine controller and are designed to detect and flag any production defects in each absorbent article being made.
The moment a check point detects and flags a production defect to the machine controller, the latter assigns that information item to a particular step 102 of the shift register 101. The step 102 is shifted along the shift register until reaching a last position 108 corresponding to the rejection station where the defective absorbent sanitary article is rejected.
At the check points, generally speaking, there are optical inspection systems, such as photocells, or vision systems comprising, for example, commercial video cameras.
A photocell is an inspection component which generates a random signal 103 which is asynchronous relative to the machine sync signal 100. The asynchronous signal 103 of the photocell is defined as ambiguous 104 when it is concurrent with the machine sync signal 100 and unambiguous 105 when it is not concurrent with the machine sync signal 100.
If the photocell signal is unambiguous 105, relative to the machine sync signal 100, the machine controller assigns the defective article information item to a particular step 102 of the shift register 101, so that the defective absorbent sanitary article is certain to be rejected when it reaches the rejection station.
If the photocell signal is ambiguous 104, relative to the machine sync signal 100, an uncertainty window is created and the machine controller is unable to assign the defective article information item to a particular step 102 of the shift register 101. In this case, the machine controller assigns the information item to two or more steps 102 of the shift register 101 in order for the defective absorbent sanitary article to be certainly identified and rejected. Unfortunately, however, that means that absorbent sanitary articles which are not defective are rejected together with the defective article at the rejection station.
From the above, it may be inferred that every time the photocell signal is ambiguous 104 relative to the machine sync signal 100, a significant quantity of good products are rejected together with the defective products, which translates as a considerable waste of raw materials.
Unlike photocells, vision systems are stable response systems since they are controlled by a trigger pulse 106 generated by the machine controller.
From the moment the vision system receives the trigger pulse 106, it has a maximum response time to flag a defect, if any, thus defining a band of uncertainty 107 which may be concurrent with the machine sync signal 100 and hence ambiguous. as illustrated in FIG. 1. Under these conditions, the machine controller is unable to determine a specific step 102 of the shift register 101 to assign the defective article information item to and, as in the case of the ambiguous photocell signal 104, assigns the information item to two or more steps 102 of the shift register 101 so that the defective absorbent sanitary article is certainly identified and rejected.
Even the use of a vision system does not solve the problem of rejecting sanitary articles which are not defective. Thus, every time the band of uncertainty 107 is ambiguous relative to the machine sync signal 100 a considerable number of good products are rejected together with the defective products.